1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell, an energy storage, and a group of loads. The fuel cell performs power generation by electrochemical reactions of reactant gases. The energy storage is capable of storing and discharging electrical energy generated by the fuel cell, and capable of being directly coupled to the fuel cell. The group of loads consume at least electrical energy generated by the fuel cell or electrical energy discharged from the energy storage. Further, the present invention relates to a method of starting operation of the fuel cell system.
2. Description of the Related Art
The fuel cell is a system for obtaining direct current electrical energy by electrochemical reactions of a fuel gas (chiefly hydrogen-containing gas) supplied to an anode and an oxygen-containing gas supplied to a cathode.
For example, a polymer electrolyte fuel cell includes a power generation cell formed by sandwiching a membrane electrode assembly between separators. The membrane electrode assembly includes the anode, and the cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. In use of this type of the power generation cell, generally, predetermined numbers of the membrane electrode assemblies and separators are alternately stacked together to form a fuel cell stack.
The fuel cell is promising in vehicle applications. For example, the fuel cell is mounted in a vehicle or the like. In the vehicle applications, power generation efficiency is high, and the exhaust gas is clean advantageously.
In the fuel cell vehicle, a hybrid power supply system has been adopted. In the hybrid power supply system, in order to assist the output responsiveness or the like of the fuel cell, an energy storage such as a battery or a capacitor (electric double layer capacitor) is used in combination with the fuel cell. In this case, at the time of starting operation of the system, if the fuel cell is operated at a low temperature, power generation of the fuel cell may not be performed efficiently.
In this regard, for example, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2004-281219 includes a fuel cell for performing power generation using a fuel gas and an oxygen-containing gas supplied to the fuel cell, oxygen-containing gas supplying means for supplying the oxygen-containing gas to the fuel cell, a fuel gas supplying means for supplying the fuel gas to the fuel cell, a secondary battery for storing and discharging electrical energy, electrical energy distribution means for supplying electrical energy generated by the fuel cell to auxiliary devices required for power generation of the fuel cell to consume the electrical energy by the auxiliary devices, supplying the electrical energy to the secondary battery for charging the secondary battery, and discharging the secondary battery to supply the discharged electrical energy to the auxiliary devices. Further, the fuel cell system includes control means for warming up the fuel cell and the secondary battery by repeating the step of controlling the electrical energy distribution means to supply electrical energy generated by the fuel cell to the auxiliary devices and the secondary battery at the time of starting operation of the system, and the step of controlling the electrical power distribution means to supply the electrical energy generated by the fuel cell and the electrical energy discharged from the secondary battery to the auxiliary devices.
According to the disclosure, at the time of starting operation of the system, it is possible to stabilize the fuel cell and raise the temperature of the fuel cell by self-heating. Also, it is possible to repeat charging and discharging the secondary battery and raise the temperature of the secondary battery by self-power generation. Therefore, the fuel cell and the secondary battery are effectively warmed up in a short period of time.
In general, in the system, in a state where the output voltage of the fuel cell is equal to the terminal voltage of the battery (the fuel cell is directly coupled to the battery), it is no longer possible to further decrease the output voltage of the fuel cell from this state. Therefore, it is not possible to collect electrical current from the fuel cell.
The energy storage such as a battery or a capacitor has characteristics in which as increase in the amount of charged electrical energy, the terminal voltage of the energy storage becomes high. In the fuel cell, the current-voltage characteristics (I-V characteristics) change depending on the temperature. In particular, at the time of starting operation of the fuel cell at a temperature below the freezing point, the I-V characteristics are significantly poor.
Thus, as shown in FIG. 8, at the time of starting operation of the fuel cell at a temperature below the freezing point, the available output power generation range is very small. After collecting only a small amount of electrical current from the fuel cell, the potential of the output voltage of the fuel cell becomes equal to the potential of the terminal voltage of the energy storage. Thus, only small current can be collected from the fuel cell. Self-heating of the fuel cell is not performed sufficiently, and continuous power generation cannot be performed suitably.